The release 10 (Rel.10) standard in 3GPP LTE-Advanced (3rd generation partnership project long-term evolution-advanced, hereinafter simply referred to as “LTE-A”) uses SU-MIMO (single user-multi input multi output) using up to four antenna ports in uplink data transmission. The system throughput can thereby be improved. A demodulation reference signal (DMRS) is also transmitted in the uplink. A cyclic shift sequence (CS sequence) which is an orthogonal sequence and a Walsh sequence (orthogonal cover code, OCC) are used as the demodulation reference signal (see FIG. 1). Inter-sequence interference can be reduced in this way. Here, a terminal generates a transmission sequence used by the terminal by applying to a basic sequence, a cyclic shift corresponding to one of cyclic shift values 0 to 7 (that is, indicated with 3 bits) reported from a base station. A cyclic shift value is represented by “cyclic shift (CS) number “0 to 11”×symbol length/12.” In this way, orthogonality among cyclic shift sequences can be secured. As OCC, two OCCs: w1=[1 1] and w2=[1 −1] having orthogonality are provided. Two pilot signals located in one subframe are multiplied by w1=[1 1] or w2=[1 −1]. A Zadoff-Chu sequence is used as a basic sequence in the LTE standard.
FIG. 2 shows a table of demodulation reference signals defined in the Rel.10 standard. In FIG. 2, four antenna port identification numbers, cyclic shift values and identification information of a Walsh sequence (that is, w1 or w2) corresponding to each antenna port identification number are associated with each other regarding each of eight demodulation reference signal candidates. When the base station assigns demodulation reference signal candidates to a terminal, the base station indicates identification information of the demodulation reference signal candidate (3 bits) to an assignment target terminal via a PDCCH (physical downlink control channel) (see Non-Patent Literature 1).
In FIG. 2, antenna ports 40, 41, 42 and 43 mean antenna ports #0, #1, #2 and #3 when four antenna ports are used. Antenna ports 20 and 21 mean antenna ports #0 and #1 when two antenna ports are used. Although antenna port 10 is not shown in FIG. 2, antenna port 10 is located on the same column on which antenna ports 40 and 20 are located.
As is obvious from FIG. 2, an offset pattern (hereinafter simply referred to as “offset pattern”) regarding offset values of cyclic shift values with respect to a second antenna port (#1), a third antenna port (#2) and a fourth antenna port (#3) from a cyclic shift amount corresponding to a first antenna port (#0) is fixed. That is, in the case of four antenna ports (that is, in the case of 4-antenna MIMO transmission), the offset pattern is “0, 6, 3, 9.” That is, CS intervals among antenna ports are designed to be maximum in both cases of 2-antenna MIMO and 4-antenna MIMO. For example, when the number of antenna ports used for DMRS transmission is 2, the amount of offset of CS numbers is “0, 6” and the CS interval becomes a “symbol length/2.” On the other hand, when the number of antenna ports used for DMRS transmission is 4, the amount of offset of CS numbers is “0, 6, 3, 9” and the CS interval becomes a “symbol length/4.”
According to the Rel.10 standard, when the number of antenna ports used for DMRS transmission is 2, DMRS is orthogonalized by only a CS sequence. For example, for one DMRS, OCC numbers are common to two antenna ports and only one of w1 and w2 is applied. Furthermore, according to the Rel.10 standard, when the number of antenna ports is three or more, OCCs differing from one antenna port to another may be applied for one DMRS. This is because, when the number of antenna ports used for DMRS transmission is two, only the CS sequence is sufficient, whereas when the number of antenna ports used for DMRS transmission is three or more, the CS interval cannot always be said to be sufficient for orthogonalization by only the CS sequence. For this reason, when the number of antenna ports used for DMRS transmission is three or more, orthogonality is improved through orthogonalization by OCC in addition to the CS sequence (see Non-Patent Literature 3). In FIG. 2, for half of the demodulation reference signal candidate group (3-bit indication is ‘000’, ‘010’, ‘001’ and ‘111’), OCC identification information is common regarding antenna ports #0 and #1, and OCC identification information differs regarding antenna ports #2 and #3. For the remaining half (3-bit indication is ‘100’, ‘011’, ‘101’ and ‘110’), OCC identification information is common to all antenna ports #0, #1, #2 and #3.
As described above, a pair of a “CS pattern candidate” and an “OCC pattern candidate” are associated with each of eight RS pattern candidates in the DMRS reference signal (RS) pattern table shown in FIG. 2. The term “CS pattern candidate” is defined by four antenna port identification numbers and a cyclic shift value (or CS number) corresponding to each antenna port identification number. On the other hand, the “OCC pattern candidate” is defined by four antenna port identification numbers and identification information of a Walsh sequence corresponding to each antenna port identification number (that is, w1 or w2).
Furthermore, according to the Rel.10 standard, SRS (sounding RS) which is a receiving quality measuring reference signal is transmitted in uplink. This SRS is time-division multiplexed (TDM) with a symbol different from DMRS. More specifically, a DMRS is mapped to the fourth and eleventh symbols and an SRS is mapped to the fourteenth symbol (see FIG. 3). Note that a DMRS and SRS are mapped independently of each other. Furthermore, DMRSs and SRSs of a plurality of terminals are assigned to the same frequency.
As described above, DMRSs are used to demodulate data signals. A terminal which is on the data signal transmitting side transmits a DMRS from an antenna port from which a data signal is transmitted in a subframe where the data signal is transmitted. On the other hand, SRSs are used to acquire receiving quality information used for frequency scheduling. Therefore, since SRS transmission has a low correlation with data signal transmission, SRSs are generally transmitted independently of other signals. Furthermore, in an LTE uplink, a bandwidth in which a demodulation reference signal is transmitted is identical to a bandwidth of a data signal, whereas a bandwidth in which a receiving quality reference signal is transmitted is not dependent on the bandwidth of the data signal.
According to the LTE-A Rel.11 standard, an expansion of SRS resources is planned in preparation for shortage of resources used for SRS transmission (hereinafter, may also be referred to as “SRS resources”) as the number of terminals that perform MIMO communication increases. A candidate for such a method is a method of code-multiplexing an SRS (e.g., for UE#1) with a DMRS (e.g., for UE#2) as shown in FIG. 4 (see Non-Patent Literature 2). According to the technique disclosed in Non-Patent Literature 2, an SRS is configured in the same way as for a DMRS (e.g., sequence group) except that trigger information used for timing control of SRS transmission is indicated by a base station to a terminal via a PDCCH. That is, when transmission of an SRS to be code-multiplexed with a DMRS is triggered, an SRS is generated according to the table described in FIG. 2 as with DMRS. Examples of the method of triggering SRS transmission include a method of changing an instruction (e.g., constellation) which is less likely to be used during normal operation to trigger information and a method of newly providing a trigger bit or the like.